A user terminal may be any kind of terminal by which a user accesses a communication network. Examples of a user terminal include a cellular phone, personal digital assistant, palmtop, laptop, desktop, gaming equipment, media player, sensor, and so on. Note that some types of communication network, such as 3GPP networks, primarily keep track of user subscriptions rather than user terminals, although user terminals are associated with subscriptions through UICC/SIM cards.
Geographical areas served by Public Land Mobile Networks (PLMNs) are typically partitioned into mobility areas (MAs) which may be referred to as, e.g., location areas (LAs), routing areas (RAs) or tracking areas (TAs). An MA consists of one or more cell radio coverage areas and a set of MAs managed by one or more nodes is known as the service area (SA) of this node or these nodes. The purpose of these areas is to keep approximate track of the whereabouts of user terminals.
The design of these MAs requires a trade off between the need for position updating (i.e., user terminals updating networks about changes in their MA) and user paging (i.e., networks locating user terminals inside their MA when there is incoming traffic). The larger the MA, the fewer resources are required for updating (large MAs mean that users have a smaller chance of crossing an area boundary to another MA) but the more resources are required for paging (a large area means that a user must be paged in more cells).
The second and third generations of mobile systems (2G and 3G, also referred to as GSM and WCDMA respectively) use a “double” partitioning; LAs for circuit switched services and RAs for packet switched services. Mobile Switching Centres (MSCs), which manage circuit switched traffic, keep track of the LAs of all user terminals in their respective SAs while Serving GPRS Service Nodes (SGSNs), which manage packet switched traffic, keep track of the RAs of all user terminals in their respective SAs. A problem with this solution is that it is inflexible in the sense that all users, irrespective of how mobile they are, must be handled in the same way. This means, for example, that those users who do not move at all must be unnecessarily paged in large areas and/or those users who move quickly between areas must update their LAs and RAs continuously.
The fourth generation of mobile systems (4G, also referred to as Long Term Evolution, LTE), which only handles packet switched traffic, uses single partitioning into TAs. Mobility Management Entities (MMEs) keep track of the TAs of all user terminals in their respective SAs. TAs are identified by numbers (TAIs) and collections of TAIs are known as TAI lists. TAI lists, which can amount to at most 16 TAs, are assigned by MMEs and increase the flexibility compared to 2G and 3G, as user terminals can be assigned TAI lists, and thus be registered in multiple TAs. The scope of TAI lists is limited to the TAIs of a specific MME SA
LTE user terminals can be described as state machines with mobility states EMM-DEREGISTERED and EMM-REGISTERED and connection states ECM-IDLE and ECM-CONNECTED. In this simplified view, a user terminal becomes EMM-REGISTERED as it is switched on, EMM-REGISTERED and ECM-IDLE when an Access Point Name (APN) is established, and EMM-REGISTERED and ECM-CONNECTED when actually transmitting or receiving data.
Inside an MME SA, the whereabouts of user terminals in ECM-IDLE are recorded to the level specified by the TAI while the whereabouts of user terminals in ECM-CONNECTED are recorded to the cell level. To this end, all cells repeatedly broadcast their TAIs and user terminals continuously tune in to the cell that currently has the strongest signal. A user terminal will perform a TA update (TAU), i.e., report its location to the MME, if it cannot find the broadcast TAI in its TAI list, or its periodic inactivity timer expires. Note that for user terminals that are ECM-connected, the network knows the cell in which the user terminal is located, while for users that are ECM-idle the network only knows that the user terminal is in a TA of their TAI list (and, in addition, it will be known in which cell the terminal was last active).
Although the notation of TAI lists in 4G introduces a degree of flexibility compared to the rigid LAs and RAs in 2G and 3G, it does not solve the problem of optimising the trade-off between MA reporting and user terminal paging. On the contrary, “replacing” LAs and RAs by TAs may be considered to be simply a matter of terminology.
The concept of a TAI list has not arisen before 4G proposals, and although some similar concepts have been used in 2G and 3G networks, such concepts would require manual and labour intensive configuration of TAI lists. The use of such, largely non-overlapping, TAI lists would also mean that the problems with high control traffic overhead at TA borders remain. Furthermore, such lists and paging sequences would, in changing network conditions, lead to inefficient paging strategies that unnecessarily use a lot of signalling, increasing paging traffic overhead and localisation times.